This invention relates to phased array semiconductive lasers having multi-emission or broad emission capabilities and in particular to phased array lasers having structural design that maintains their operation in a preferred single lobe far field pattern.
Phased array semiconductor lasers comprise a plurality of closely coupled or spaced emitters on the same internal structure of substrate. Examples of such phased array lasers are illustrated in U.S. Pat. No. 4,255,717, now U.S. Pat. No. Re. 31,806, issued Jan. 15, 1985, and in an article of William Streifer et al., entitled "Phased Array Diode Lasers", published in the June 1984 Issue of Laser Focus/Electro-Optics. The emitters of such a laser are confined by the periodically spaced current confinement means, e.g. stripes, for current pumping and establishment of spaced optical filaments in the active region of the structure or by internal waveguide structuring. The current confinement means may be interconnected or the emitters closely spaced to a degree that the optical mode established in each of the filaments couples to neighboring optical filament modes, i.e., the evanescent wave overlaps into adjacent optical lasing cavities. The array of optical fields produced become phased locked, and, if the phase difference between adjacent current confinement means is zero, the lateral radiation pattern in the far field will comprise a single lobe. However, as explained in the abovementioned article, the phased array laser does not operate to radiate in a single lobe but rather generally operate with radiation into two or more lobes in the far field pattern. The phase relationship between adjacent optical filaments is not under control and the phases themselves adjust in a manner to minimize laser threshold current. In most cases, it appears that lasing is favored in a supermode wherein the optical field between adjacent optical emitters passes through zero. This is because in most real refractive index lasers as well as many gain guided lasers, pumping gain is reduced at locations between the laser filaments or emitters.
The foregoing explanation is exemplified by reference to FIG. 1. FIG. 1 is a schematic illustration of an array of N coupled emitters wherein, in the particular case shown, N=10. An array laser with N coupled emitters has N possible coupled modes which are referred to as "supermodes". A supermode is a cooperative lasing of the N optical emitters or filaments of the array laser. Since there are N emitters, there are N possible supermodes since all these emitters are optically coupled together.
Each supermode has the property that the 1.sup.st and the N.sup.th supermode have the same intensity pattern or envelope, the 2.sup.nd and the N-1.sup.th have the same intensity envelope, and, in general, the i.sup.th and N-i.sup.th have the same intensity envelopes.
FIG. 1A shows the supermode field amplitude pattern for a ten emitter or element array laser wherein i=1, i.e., the 1.sup.st or fundamental supermode. The 1.sup.st or fundamental supermode has all emitters lasing in phase with an amplitude distribution representative of half a sinusoidal cycle. This is the only supermode pattern that radiates in a single central lobe in the far field pattern because all emitters radiate in phase.
FIG. 1B shows the supermode field amplitude pattern for the N.sup.th supermode which, for this particular example, is i=10. The amplitude pattern is very similar to the amplitude pattern shown for the 1.sup.st supermode in FIG. 1A except that alternating emitters have alternating phase, i.e., are out of phase by .pi.. As a result, this supermode will radiate in two fairly symmetrical lobes in the far field pattern.
There are eight other supermodes for i=10. The supermode field amplitude pattern for the 2.sup.nd supermode is shown in FIG. 1C wherein the amplitude envelope across the array is sinusoidal comprising one positive half cycle and one negative half cycle. The 2.sup.nd supermode will lase in two closely spaced symmetrical lobes in the far field pattern.
Thus, for a uniformly spaced array of identical emitters, the 1.sup.st and N.sup.th supermode envelopes are half a sinusoidal period, the second and the N-1.sup.th supermode envelopes are two half sinusoidal periods, etc. The phases of the individual emitters in the 1.sup.st and N.sup.th supermodes differ. Specifically, for the 1.sup.st supermode, all emitters are in phase and for the N.sup.th supermode, the phases alternate between zero and .pi.. Usually the 1.sup.st and N.sup.th supermodes have the lowest current thresholds as compared to all other supermodes because their intensity envelopes do not exhibit nulls near the center of the array where the charge density is greater as a result of current spreading and charge diffusion in the active region of the array laser. However, as previously indicated, the N.sup.th supermode, which radiates in two lobes, has a lower current threshold of operation than the 1.sup.st supermode due to the lower gain which naturally occurs between emitting regions.
Phased array lasers have high utility due to their high power output. It is preferred that the power be concentrated in a single lobe, i.e., in the 1.sup.st supermode. The reason is that a substantial majority of all laser applications require power in a single far field lobe. If radiation is experienced in more than one lobe, measures are taken to diminish or otherwise attempt to eliminate or block off the other operating lobes.
In the article of William Streifer et al, "Channeled Substrated Nonplanar Laser Analysis Part 1: Formulation and the Plano-Convex Waveguide Laser", IEEE Journal of Quantum Electronics, Vol. QE-17(5), pp. 736-744, May, 1981, a single stripe laser with a planar active region adjacent to a waveguide laser of spatially lateral varying thickness is analyzed. Looking at FIG. 2 of this article, the equivalent real refractive index is greatest in the central region of the active layer where the adjacent waveguide layer is at its thickest portion width, i.e., at the center of the waveguide layer, and therefore provides real refractive index waveguiding in the same region of both the active and waveguide layer.
However, the filling factor, .GAMMA., which is the percentage of optical modal intensity in the laser active region, is at a minimum at this point, i.e., at the point of maximum index. Since local gain, .gamma.(y), is proportional to the filling factor, .GAMMA., local gain will also be lower at this point. This is illustrated in FIG. 3 of the cited article wherein the two curves show the equivalent refractive index, n.sub.eq, and the filling factor, .GAMMA., versus latent position along the active region of the laser shown in FIG. 2 of the article. To be noted is that the two curves are practically opposite complements of one another. Thus, those regions of highest gain in the active region would be those areas offset or adjacent to the center of the active region.
In the case of a multi-emitter laser, the intensity I(y) for the 1.sup.st and N.sup.th supermodes will respectively take on the intensity envelopes illustrated in FIGS. 2A and 2B and the corresponding gain profile .gamma.(y) will have the envelope shown in FIG. 2C. The intensity envelope for 1.sup.st supermode in FIG. 2A illustrates the higher overall intensity experienced as compared to the intensity pattern for N.sup.th supermode. To be noted is that the local gain peaks of each of the emitters coincides with the i.sup.th and N.sup.th mode peaks.
In the case where the emitters of the phased array laser are real refractive index waveguide lasers, the overall gain of the 1.sup.st and N.sup.th supermodes are given by the equations: ##EQU1##
Since I.sub.N is relatively more concentrated in the regions of the waveguides where local gain is a maximum, the overall gain of I.sub.N mode exceeds that of the I.sub.1 mode and the I.sub.N mode, as a result, lases at a lower pumping current. As a result, the N.sup.th supermode will be preferred since it has a lower threshold of operation than the 1.sup.st supermode. Unfortunately, the N.sup.th supermode in most cases radiates in two lobes, as previously mentioned.
Recently, there has been much activity relative to phase locked array lasers or phased array lasers where efforts have been established to discriminate among the supermodes and provide fundamental supermode selection (N=1). One such suggestion was at the IEEE 9.sup.th International Semiconductor Laser Conference in Brazil, July, 1984 wherein J. Katz et al. presented a talk on supermode discrimination by controlling lateral gain distribution along the plane of the lasing elements by incorporating a separate contact to each laser array element and tailoring the currents through the array laser elements. The abstract for the talk is found in the Proceedings of the Conference at pages 94 and 95 entitled "Supermode Discrimination in Phase-Locked Arrays of Semiconductor Laser Arrays".
What is needed is to somehow design a phased array semiconductor laser so that the gain, g.sub.1, of the preferred 1.sup.st supermode will exceed the gain, g.sub.N, that of the multi-lobe N.sup.th supermode of such phased array lasers. This is graphically illustrated in FIG. 3 wherein in a phased array semiconductor laser of this invention, the desired intensity envelope of the 1.sup.st supermode is shown in FIG. 3A, the intensity envelope of the N.sup.th supermode is shown in FIG. 3B and the corresponding gain profile, .gamma.(y), is shown in FIG. 3C. To be noted is that the gain peaks have each been shifted one half period relative to the corresponding intensity peaks of the laser's points of high emission intensity in connection with all supermodes so that the gain peaks of such points coincide with the trough or valley points of the supermode intensity pattern.
It is, therefore, the object of this invention to provide phased array semiconductor laser structures that are designed to shift the gain relative to the optical guide of the laser emitters so that such laser structures will continuously operate in the preferred 1.sup.st supermode.